1) Field of the Invention
The present invention relates to an image forming apparatus, an image forming method, and a fixing unit, and more particularly, to an image forming apparatus like a copying machine, a digital multifunction product, and a printer that apply heat to a fixing member of a fixing unit using an auxiliary power supply, an image forming method, and a fixing unit that is used in the image forming apparatus.
2) Description of the Related Art
The image forming apparatus such as the copying machine or the printer forms an image on a recording medium like plain paper or an OHP transparency. In this image formation, an electrophotographic system is adopted taking into account high speed of the image formation, an image quality, cost, and the like. The electrophotographic system is a system in which a toner image is formed on a recording medium and fixed on the recording medium with heat and pressure. As a fixing system, a heat roll system is adopted most frequently at present in terms of safety and the like. The heat roll system is a system in which a heating roller, which is heated by a heat generation member like a halogen heater, and a pressurizing roller, which is arranged to be opposed to the heating roller, are brought into pressed contact with each other to form a mutual press-contact section called a nip section, and the recording medium having the toner image transferred thereon through is passed through this nip section and heated.
In recent years, environmental problems have increased in importance, and efforts have been made to reduce energy consumption in the image forming apparatus such as the copying machine or the printer. What cannot be neglected in considering the reduction of energy consumption in the image forming apparatus is power saving in the fixing unit that fixes toner on a recording medium.
In realizing power saving, it is effective to reduce energy consumption when the fixing unit is on standby. Thus, it is desirable to reduce power supply to zero when the fixing unit is not used. However, in a conventional structure for the fixing unit, if power is reduced to zero when the fixing unit is on standby, it takes time to warm up a heating roller thereof when the fixing unit is used again. This makes waiting time longer and deteriorates convenience for a user. Consequently, a structure for increasing temperature of the heating roller rapidly has been required.
To reduce the warm-up time for the heating roller, it is effective to increase input energy per a unit time, that is, rated power. Actually, in some high-speed image forming apparatuses with high print speed, a supply voltage is increased to 200 volts to increase the rated power. However, in typical offices in Japan, in general, a power supply has an upper limit of 100 V/15 A (1500 W), and it is necessary to apply special work to power supply related facilities in a place where an image forming apparatus is set. Thus, it cannot be said that this is a general solution. Therefore, the fact of the matter is that, even if it is attempted to warm up the heating roller in a short time, the upper limit of the input energy cannot be lifted.
In addition, there is a method of reducing thickness of the heating roller to warm up the heating roller in a short time. However, since a thermal capacity of the heating roller decreases, in the case of an image forming apparatus with high print speed, temperature may fall if continuous printing is performed even in a state in which temperature on the surface of the heating roller has risen to a set temperature.
To improve the warm-up of the heating roller, power is accumulated in an auxiliary power supply in advance and the power is supplied from the auxiliary power supply at the time of power shortage. This makes it possible to solve the problem of the fall in temperature at the time of a print operation.
For example, Japanese Patent Application Laid-Open Publication No. Hei 10-282821 proposes a technique for increasing maximum supply power using an auxiliary power supply in a fixing unit to thereby realize reduction in energy consumption. Such a fixing unit disclosed in Japanese Patent Application Laid-Open Publication No. Hei 10-282821 supplies power from a main power supply unit and a secondary battery or a primary battery and uses a nickel-cadmium battery or a lead-acid battery as a source of the secondary battery. Such a secondary battery has a characteristic that a capacity thereof deteriorates and decreases when charge and discharge are repeated many times and a useful life thereof is reduced when discharge is performed with a larger current. In general, even in the nickel-cadmium battery, which is said to have a long useful life with a large current, the number of times of repetition of charge and discharge is about 500 to 1000. If charge and discharge are repeated twenty times a day, the battery comes to the end of its life in about one month. Therefore, the secondary battery has a disadvantage that time and labor are required for replacement of the battery, and running cost such as cost for the battery to be replaced increases. Moreover, a lead-acid storage battery is not preferable as office equipment because, for example, the battery uses liquid sulfuric acid.
Consequently, for example, Japanese Patent Application Laid-Open Publication No. 2002-184554 discloses an image forming apparatus that uses a large-capacitance capacitor like an electric double layer capacitor as an auxiliary power supply for a fixing unit. Such an electric double layer capacitor has a characteristic that the number of times of repetition of charge and discharge is several tens thousand to several hundreds thousand, and a useful life depending on the number of times of charge and discharge is far longer than that of a battery.
However, when the auxiliary power supply is used, power must be charged in the auxiliary power supply. If a print operation is performed in a state in which sufficient power is not charged in the auxiliary power supply, charge in the auxiliary power supply is fully consumed in the midstream of the print operation. As a result, print speed falls from the midstream of the print operation, and if power of the auxiliary power supply runs short, an image with a poor fixing property is obtained unless fall in temperature in the heating roller is prevented by taking measures such as increasing an interval of print operations. On the other hand, it is preferable for a user that time until completion of an image forming job is shorter.
There is also a technique for changing a linear velocity or a sheet feeding time according to a state of a charging voltage at an auxiliary power supply. However, in any event, longer time is required until completion of a print operation compared with a case in which a fully charged auxiliary power supply is used.